How they could of reblogged your content
When some one likes your post via your site using the admin bar they are presented wit the option to reblog the content they click liked on please not that the reblog option only uses a excerpt from your article...
Option B for reblogging content
Press This is a bookmarklet: a little app that runs in your browser and lets you grab bits of the web.
Use Press This to clip text, images and videos from any web page. Then edit and add more straight from Press This before you save or publish it in a post on your site.
Drag-and-drop the following link to your bookmarks bar or right click it and add it to your favorites for a posting shortcut.
If someone clicks on the link to your blog from their blog a hit should show up in your stats that would be the only way to find out who shared your content
The press this bookmarlet can be found via the left navigation panel under the category Tools via your dashboard